Possible null pointer dereference - otherwise it is redundant to check it against null

Question

I have the following code, which is working properly:

int result = ERRORCODE_OK;
if (dataObj == NULL || dataObj->inputSignal == NULL)
{
   result = ERRORCODE_MISSING_DATAOBJ;
}
if (result == ERRORCODE_OK && dataObj->spectrum == NULL) // CPP-Check error
{
   result = Calculate(dataObj->inputSignal, .. );
} 
return result;

But CppCheck gives me the following error:

Possible null pointer dereference: dataObj - otherwise it is redundant to check it against null.

I don't understand why. If the dataobj is NULL, then the result will be something else then ERRORCODE_OK.

Answer 1

CppCheck doesn't inspect deep enough to see that your second condition won't be fully evaluated if the first one succeeds:

int result = ERRORCODE_OK;
if (dataObj == NULL || dataObj->inputSignal == NULL)
    result = ERRORCODE_MISSING_DATAOBJ;

// Here, you can see that either dataObj!=NULL or result!=ERRORCODE_OK.
// But CppCheck can't!

if (result == ERRORCODE_OK && dataObj->spectrum == NULL)
    result = Calculate(dataObj->inputSignal, .. );
return result;

Three alternative ways of pacifying the checker present themselves. Firstly, just repeat the check that dataObj is non-null in the second if. Secondly, change the second if to else if:

int result = ERRORCODE_OK;
if (dataObj == NULL || dataObj->inputSignal == NULL)
{
    result = ERRORCODE_MISSING_DATAOBJ;
}
else if (result == ERRORCODE_OK && dataObj->spectrum == NULL)
{
    result = Calculate(dataObj->inputSignal, .. );
} 
return result;

Thirdly, return as soon as you find one of the error cases:

if (!dataObj || !dataObj->inputSignal)
    return ERRORCODE_MISSING_DATAOBJ;
if (dataObj->spectrum)
    return ERRORCODE_OK;
return Calculate(dataObj->inputSignal, .. );